Short Wave - Emotions — They're Not Just For Humans

Episode Date: May 4, 2022

Scientists have discovered the underpinnings of animal emotions. As NPR brain correspondent Jon Hamilton reports, the building blocks of emotions and of emotional disorders can be found across lots of animals. That discovery is helping scientists understand human emotions like fear, anger — and even joy. Express your joy, fear and fine — even your scientific rage to us. We're at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. So, John, I can tell that the warm weather has arrived because I woke up this morning to, like, a small swarm of fruit flies in my kitchen. And I just, like, I couldn't swat them fast enough. Oh, whoa, stop right there. What? Because I am here today to tell you how these tiny critters you're talking about, how they can teach us something important about mental health conditions, like post-traumatic stress disorder. Whoa, what? Are you our highly evolved brain guy saying that fruit flies actually get PTSD?

No, I am not making that claim. But human emotions like fear and anxiety, they seem to have evolved from brain circuits that you can find in animals. And I've been talking to scientists who say that what you might call the building blocks of emotions and of emotional disorders, they can be found not only in fruit flies, but lots of other animals. So one guy I talked to is Dr. Kerry Ressler. a psychiatrist at Harvard Medical School, McLean Hospital. He told me PTSD involves an emotion that is meant to keep us alive, fear. We really see PTSD as a disorder in which this evolved, important fear response is essentially gone too far.

And so it's really an almost unhinged threat or fear emotion that is very hard to control and that takes over people's lives. Right, because the traumatic experiences have changed their brain. But can you really see that sort of change in an animal? Carrie is one of the scientists who think you can. Take a mouse, for example. Carrie told me about a study that compared typical mice to mice that had experienced trauma. The typical mice learned to associate a particular tone with a mild electric shock,

and then they would freeze when they heard it. But when the tone was no longer accompanied by a shock, these mice figured out pretty quickly that they were now safe, and they stopped freezing. Carrie says trauma change that whole learning curve. If the animals had prior trauma, they'll learn that more quickly. They'll freeze for longer, and it takes them longer to extinguish or learn that now that tone is actually safe. So what Carrie Wrestler is saying is these mice are acting a bit like someone with PTSD. How does that help scientists understand how to help a person with this disorder?

Two words, Aaron, brain circuits. There is pretty good evidence that the same circuit. that predispose a mouse to freezing are overactive in people with PTSD. So scientists are hoping that they can find a way to sort of calm down that circuit and do something for people with a disorder. Today on the show, how discovering the underpinnings of animal emotions is helping scientists understand human emotions. Like fear, anger, and even some positive emotions, including joy.

I'm John Hamilton. I'm Aaron Scott, and you're listening to Shortwave, the Daily Science Podcast, from NPR. So, John, we're talking animals, emotions, and evolution. What makes scientists think that emotions evolved? Couldn't they just be something we learn growing up? There are certainly psychologists who see it that way. And they have a point. Humans are probably not born with a highly developed sense of, say, regret or shame. But biologists argue that we are born with a brain that's wired for emotions. And versions of that wiring have been around for millions of years in other species.

One of the main scientists who makes this argument is David Anderson at Caltech. He goes through all of it in his latest book, which is called The Nature of the Beast, How Emotions Guide Us. Emotions are brain functions that evolved over time by natural selection. They didn't just appear on the planet with the advent of Homo sapiens. All right. So emotions evolved. But I thought this whole idea of other animals experiencing emotions was controversial. I mean, how do you really know what sort of emotion of fruit fly is experiencing? You don't. When people talk about emotions like sadness or surprise, we are describing a subjective

experience, a feeling, and animals can't tell us what they're feeling. I know that seems pretty obvious when you're talking about fruit flies, but, you know, maybe not so much when it comes to our pets. So, Aaron, I want you to listen to this cat and tell me how it's feeling. Oh, yes, this is a happy cat, ain't no question about that. I agree. I mean, we all know that cats are happy when they purr and angry when they hiss. Except, we don't. We really have no idea whether our own subjective perception of happiness has anything to do with what a purring cat is feeling. I should mention here that David himself has two cats named Buster and Serafina, and when he's at home, he lets himself assume that he has a deep understanding of their emotional lives.

As a neuroscientist, I'm looking for objective evidence. And for that, we have to do more than just project our own emotions onto other animals, because animals are not little people in furry costumes. David told me that in order to study emotions in animals, scientists need to set aside their own perceptions of emotional states. In other words, they need to look beyond feelings. Emotions are like a huge iceberg in the brain, and the feeling part's just the tip of the iceberg above the sea of our consciousness.

The part below is what we share in common with animals. And so what my colleagues and I have been trying to do is to figure out when an animal's behavior reflects what we think of as an internal emotion state. Okay, so how is an emotion state different than a feeling? Take fear, for example. The feeling is the part I'm aware of. It's the subjective experience that makes me say, I'm afraid. Okay. But beneath that feeling is an emotion state, a brain state that I may or may not even be aware of.

It causes my heart rate to increase, my palms to sweat, and it prepares me to react instantly if I sense any kind of threat. The feeling is fear, but the way I'm acting is what a scientist would call defensive arousal. And the behaviors I'm exhibiting, along with the brain circuits that control them, those are the part of fear that scientists can study in me or any other am. animal. Even a fruit fly. So it seems. I mean, let me give you an example from David's lab. They have this enclosed arena where they do fruit fly experiments. And one of the experiments they've done involves casting a shadow to make it look like a predator is flying overhead. Of course, when the flies see that shadow, they start jumping around trying to escape. But that could just be a reflex, you know, a programmed response to a threat. What's interesting is that the more times

the flies see the shadow, the more jumpy they get, and the longer they keep jumping after the shadow is gone. And David told me that's pretty similar to the way he would react to a deadly threat. If I were hiking in the San Gabriel's near where I live and I heard a rattlesnake by the side of the trail, I'd jump in the air, but even for minutes after the snake had slithered away into the bushes, my heart would be pounding, my mouth would be dry, and I'd probably jump every time I saw a snake-like object in front of me, if it was a stick. So in both David and the fruit flies, a threat has caused some sort of emotion state. The defensive arousal you were talking about. Yeah. Okay. So the behavior is similar, but what about the brain circuits? Can we really compare a fruit fly brain to a human brain? In some ways, yes. At least when

it comes to the brain state that underlies what we would call fear. You can trace this throughout evolution. So mice, for example, have a brain circuit that's a lot like the one that produces defensive arousal in fruit flies. Mice seem to have these specialized brain cells that become very active when an animal detects a threat and gradually return to normal after the threat is passed. And it turns out these specialized brain cells are also found in humans. Can any other emotions be traced to animals? Yes. Another one is anger. Of course, when I say anger, I'm talking about the feeling I get when, you know, some driver cuts me off in traffic. But what seems to lie beneath that feeling is a brain state that predisposes animals like me to aggressive behavior.

In my case, I want badly to express my displeasure by extending the middle finger of my right hand. Also, I'm sure, deep in our evolutionary history that. Yeah, clearly it goes back millions of years, the middle finger. You know, I talk to a scientist who is an expert on that sort of behavior. Her name is Diyu Lynn, and she's at New York University, and Diyu has spent most of her career studying the brain areas involved in aggression. She told me she's found one that appears to be critical. It's a tiny, tiny region, very deep in the brain, and we all have it. In people, it's near the bottom of the hypothalamus, so that's just above the pituitary gland.

And Diyu told me that in mice and other animals, this clump of brain cells is part of what she calls a core aggression circuit. Switch it on and a mouse will attack, switch it off, and even the animal's natural aggression vanishes. So obviously you can't do the same experiment in people, but Daew says doctors sometimes use deep brain stimulation to deactivate the aggression in extremely violent psychiatric patients. When the patient is so aggressive and a uter that aggression is uncontrollable, that's sort of the loss of resort. Wow, that's a reminder. We can just turn it off. But, John, so far we've been talking about negative emotions, and I'm pretty sure that you promised me some joy in this podcast, please?

I did. And the joy involves an experiment I talked about on a previous podcast. So there's a team of researchers at the University of Pittsburgh who think they found a brain circuit in people and monkeys, and this brain circuit becomes active when they are experiencing joy. The hope is that by stimulating the circuit with electrical pulses, they can produce the same effect. So there you go, Heron. Joy, at the touch of a button. Wire me up. Thank you very much for bringing this to us, John. Anytime.

You can find a link to the episode about Joy in the show notes. This episode was produced by Rebecca Ramirez, edited by Giselle Grayson, who is also our senior supervising editor and fact-checked by Margaret Serino. Neil Carruth's, the senior director, director of on-demand news programming, and Anya Grumman is our senior vice president of programming. I'm Aaron Scott. Thank you for listening to Shortwave, the Daily Science Podcast from MPR.